1. Field of the Invention
This invention relates to X-Y addressable solid state image sensing arrays, and more particularly to novel image sensing elements in such arrays.
2. Description of the Problem
FIG. 1 shows a prior art image sensing array 10 of the type known as a charge injection device (CID). Depending upon the design, the device can be used to sense radiation in the visible or infrared portions of the spectrum. Briefly, CID 10 comprises an X-Y addressable array of pairs of adjacent MOS capacitors 11 and 12. The pairs of MOS capacitors comprise the image sensing elements of the array. The array is scanned in the X direction by a horizontal scan register 14; and in the Y direction by a vertical scan register 16. One pair of MOS capacitors is disposed at the junction of each horizontal and vertical scan line. One capacitor of each pair (e.g. capacitor 11) is connected to a horizontal scan line, and the other (e.g. capacitor 12) to a vertical scan line. As long as at least one of the MOS capacitors is biased "on" by one of the scan registers, a photogenerated minority charge can be stored beneath that capacitor. The photocharge can be shifted back and forth at will between the capacitors by always keeping at least one capacitor biased "on" and biasing the other capacitor "off." When both capacitors are biased "off," the photocharge is injected into the semiconductor substrate where it recombines to generate a readout current in the substrate. Early CID image sensors sensed this readout current as the packets of photocharge were injected one by one from the image sensing elements of the array into the substrate. Unfortunately, the recombination process is slow. Injected charges spread into neighboring sensing elements, and signals from consecutively readout elements are mixed together. To overcome these problems, special charge collectors were built into the CID image sensors to speed up the recombination process and to limit the lateral dispersion of the injected charges. Another problem faced with CID image sensors is the limitation on signal-to-noise ratio that arises due to the large capacitance of the input mode.
In a readout technique, called "parallel injection" the operation of sensing the image charge on one of the address lines and injecting the charge into the substrate are separated. The photocharge at each sensing site is detected during a line scan by transferring the charge from one capacitor to the other. Multiple readouts of the same charge pattern are possible with the technique, however, the load capacitance of the address line is still the limiting factor in signal-to-noise ratio achievable by the sensor. The inventor was faced with the problem of how to further increase the signal-to-noise ratio in a CID type image sensing device while maintaining the desirable features of X-Y addressability and nondestructive readout.